Solar panelsHow do they work and how to choose them ?

Solar panels' sales significantly increase in recent years and their prices have become very attractive. So today, we can find dozens of solar panels brands, some excellent, others less.

A good solar panel will work well until a couple of decades but it is not uncommon to find panels that are already ocidized or damaged after few years.

The return on investment is not so sure.

In Madagascar, just in Antananarivo, there are at least twenty outlets, most offering cheap solar panels from China. These may possibly be the case on a small installation, but we generally quickly realize that the cheapest become often the most expensive.

What are solar panels ?

The function of the solar panels is t oconvert sunlight into electricity to charge batteries, at least in isolated as is our case here in Madagascar or in most African countries and developing countries.

You can then use the energy stored in the batteries when it is needed, often in the evening.

The batteries can use the sun's energy at when the sun is not present.

What types of panels do exist ?

You can choose between three panels types :

Amorphous silicon

Monocrystalline (background in photo)

Polycrystalline (foreground in photo)

Amorphous silicon panels are composed of large cells in all lengths. The amorphous silicon called A-si, is recognized by a matte black appearance.

These panels provide smaller powers. We often found them from 5 to 20 watts peak (Wp) as they need to be much larger than their cousin to an equal power.

They are the ones that we can find on calculating machines for example, sunlamps, or in small solar chargers like commando panels.

Mono and polycrystalline did not have, for simplicity, any difference today because their yields are almost identical. However, we can differentiate them physically by the color blue (silicon layers) panels polycrystalline types.

They are the ones that can be found for significant power from 50 watts peak (Wp) to over 200Wp

You must be careful to choose well panels, there are two types :

Those for isolated site that have a consistent voltage with that of batteries, their open-circuit voltage (Voc) close to 20V.

Those for network injection with a much higher voltage. They are used in developing countries mainly in the creation of islanded network (see page islanded network) or when using a MPPT (used in Western countries). Their open circuit voltage (Voc) is close to 70V.

Between these three technologies, the performance differences are minimal. Everythin you need for amorphous silicon panels, it is a little more space on your roof.

But any roof, even a small house can accomodae tens or hundreds of watts no problem.

How to choose ?

If you do not have a large budget, you have a small consumption and some space, you can opt for amorphous panels. This technology is about 1.5 times cheaper..

In isolated site, you prefer panels for battery charging. Indeed, their voltage is suitable for charging batteries, their open circuit voltage (Voc) must be at least 18V.

Let's take a look to the plate of a module to give you an example.

.... but 18V must be a minimum because with the heat and the Joule effect, voltage has a tendency to decrease. But to charge your batteries up to about 14, your panel should be able to produce a higher voltage.

Be careful !

Do not buy panels with less than 16V or greater than 20Voc, they are not suitable for conventinally charging batteries but need special controllers, regulators MPPTs for "Maximum Power Point Tracking", which are reserved for large installations because they are much more expensive.

On this plate, we can find the open circuit voltage (Voc), ie when the panel is not connected so a minimum of 18V.

The maximum amperage (Isc) that you will never get but that is the amperage when the panel is short-circuited.

Then the max power (Pmax), which is expressed in watts, she called Watt Peak, most manufacturers of panels show a tolerance of + or - 10% of this value.

This means that for a 100Wp panel, manufacturers tell us that their panel's power will be between 90 and 110W which of course happen musch less often. Some brands take advantage of it and panels give systematically less than with voltmeter. At the time of purchase it is important to check with the supplier. The best thing to do is to ask a test measurement, this will allow you to check the level of competence at the same time.